1. Field of the Invention
The invention relates to a multifocal lens for vision-corrective eyeglass and an eyeglass lens therewith.
2. Description of Related Art
Because a multifocal lens provided with multiple visual field areas having different refractive powers, for example, a distance-vision area and a near-vision area, can obtain visual fields having different refractive powers with a single lens, it can be used as an eyeglass lens for correcting vision such as aging vision, and the like. Furthermore, one type of multifocal lens is a progressive multifocal lens provided with visual field areas in which the refractivity changes progressively. Because there are no boundaries in the visual field areas, a progressive visual field can be obtained. Furthermore, because it is superior also in external view, it is widely used as an eyeglass lens. FIGS. 11A and 11B show the general structure of a conventional progressive multifocal lens which is widely used as an eyeglass lens. FIG. 11A is an elevational view that shows the progressive multifocal lens 1 is provided with a distance-vision area 11 for viewing objects at a far distance, and a near-vision area 12 for viewing objects at a near distance, the refractive power of which is different from that of distance-vision area 11. The near vision area 12 is provided below distance-vision area 11. Also, the distance-vision area 11 and near-vision area 12 are connected smoothly by a progressive area 13, which is a visual field area for viewing objects at an intermediate distance. The progressive area 13 is endowed with a refractive power that changes continuously.
FIG. 11B is a sectional view and shows that in a one-piece lens 1 used for an eyeglass, there are two surfaces, being a refractive surface 3 on the side of the eye and a refractive surface 2 on the side of the viewed object. It is necessary to provide these surfaces with all the performance required for an eyeglass lens, for example, a vertex power meeting the user's prescription, a cylinder power for correcting astigmatism, an addition power for correcting aging vision, and furthermore a prism power for correcting skew. Therefore, a conventional multifocal lens 1 includes a distance-vision area 11 and near-vision area 12, and the surface power is adjusted by changing the curvature of refractive surface 2 on the side of the object. A progressive multifocal lens further includes a progressive area 13. Also, a toric surface is provided on the refractive surface 3 on the side of the eye when correction of astigmatism is necessary. For simplicity, the explanation is given below, assuming a progressive multifocal lens that does not perform correction of astigmatism.
The astigmatic aberration obtained with a conventional progressive multifocal lens is shown in FIG. 12. The progressive refractive surface 5 provided on the surface 2 on the side of the object is a non-spherical surface so as to change continuously the surface refractive power. Thus, curvature changes according to each area of the surface. For example, a schematic configuration of a progressive multifocal lens in which the refractive power of distance-vision area 11 is 0.00D and the addition power Add is 3.00D. However, when the average surface power D11 of distance-vision area 11 is set to 4.00 diopters (D), the average surface power of near-vision area 12 becomes 7.00D. Consequently, on surface 2 on the side of the object, an astigmatic aberration is caused because a difference of curvature is created between the x direction (the direction that is horizontal when the eyeglass is worn) and the y direction (the direction that is vertical along the lens perpendicular to the x direction), crossing from distance-vision area 11 to near-vision area 12. Meanwhile, surface 3 on the side of the eye may be a spherical surface having a constant curvature. The progressive multifocal lens 1 of the present example may include a spherical surface endowed with an average surface power D21 which is the same as the average surface power of distance-vision area 11, namely, 4.00D. Therefore, the surface on the side of the eye has a constant curvature in the x and y directions, and fundamentally does not cause an astigmatic aberration. Consequently, in lens 1 shown in FIGS. 11A and 11B, the astigmatic aberration of the entirety of the lens is the same as the astigmatic aberration of surface 2 on the side of the object. Astigmatic aberration is represented in diopter (D) units, and the drawing of astigmatic aberration shown in FIG. 12 has the regions of specific diopters connected by contour lines. In the present specification, average surface power indicates the surface refractive power in the vicinity of the main line of sight. Average surface power D11 of the distance-vision area of the surface on the side of the object is the average surface power in the vicinity of main line of sight 14 of distance-vision area 11 of the surface on the side of the object. Also, average surface power D12 of the near-vision area indicates the average surface power in the vicinity of main line of sight 14 of near-vision area 12 of the surface on the side of the object.
A user not having astigmatism can obtain clear vision without perceiving so much the fading of an image if the astigmatic aberration appearing in the lens is 1.0 diopters or less, preferably 0.5 diopters or less. Therefore, in progressive multifocal lens 1, a comparatively wide clear-vision region 21 having an astigmatic aberration of 1.0 diopters or less, or preferably 0.5 diopters or less, is placed in distance-vision area 11 in which the range of eye movement is great. Furthermore, the required clear-vision region following this main line of light 14 is secured by making the difference of curvature of the x direction and y direction substantially zero in the vicinity of main line of sight 14, which extends from distance-vision area 11 toward near-vision area 12, bending somewhat on the side of the nose and crowding the vision. Also, an eyeglass lens 9 is formed by globe processing lens 1 into a shape matching an eyeglass frame, and is provided to the user.
In a multifocal lens such as a progressive multifocal lens, and the like, jumping and warping occurs more easily as the prescription for correcting vision is greater. Furthermore, if the addition power Add, which indicates the difference of refractive power between the distance-vision area and the near-vision area, is great, jumping and warping of images becomes even greater because the difference of curvature between the distance-vision area and the near-vision area differs greatly. In a progressive multifocal lens, the astigmatic aberration appearing in the lens also becomes greater, and the clear-vision area becomes narrower because the progressive refractive surface is made even more non-spherical. Also, in the region where the astigmatic aberration varies greatly, a comfortable visual field cannot be obtained because the images warp and jump following the movement of the line of sight. Therefore, a progressive refractive surface that should provide the user with a comfortable visual field is improved by improving the shape of the progressive refractive surface, by removing the region in which the astigmatic aberration appears greatly from the commonly used regions of the lens, and by preventing sudden variation of the astigmatic aberration.
Furthermore, in a multifocal lens, jumping and warping of images is also caused by differences in refractive power (power) between the distance-vision area and the near-vision area. That is, distance-vision area 11 has a refractive power whereby the focus meets in the distance, meanwhile, near-vision area 12 has a refractive power different from that of distance-vision area 11 whereby the focus meets nearby. Consequently, the magnifications also are not the same, and when a progressive area 13 is provided, it causes the acquired images to jump and be distorted when the eyeglass is worn because the magnification gradually varies in the progressive area 13.